The present invention relates to an antenna unit suitable as a main antenna for receiving FM broadcast signals in an automobile, or a so-called "sub-antenna" in an antenna diversity receiving system. More particularly, the invention relates to an antenna unit whose antenna element is mounted on a glass plate of an automobile.
A typical example of a conventional FM broadcast signal receiving antenna for an automobile is a whip antenna adapted to be installed on the front part, the roof, or the trunk of an automobile, as indicated at 17, 18 or 19 of FIG. 4. Recently, however, an antenna unit which is installed by the utilization of the rear window glass plate has been extensively employed.
In one example of an antenna unit of the latter type, as shown in FIG. 5, a heating wire 20 provided on the rear window glass plate for removing moisture, frost, etc., from the latter is also used as the antenna element. In another example, an antenna element 21 is provided on the portion of the glass plate where no heating wire is located. In a further example, the above-described two methods are employed in combination.
It has been disclosed in Dempa Shimbun (Radio Wave Newspaper), "High Technology", July 5, 1984, that a variety of antenna patterns have been applied to the front, rear and side window glass plates of an automobile.
Of the above-described conventional antennas, the whip antenna is excellent both in sensitivity and in directional pattern. However, since the whip antenna is, in general, maintained extended to about one meter outside the automobile, when the automobile passes through a car wash, it is necessary to retract or remove the antenna in advance. Furthermore, the whip antenna is disadvantageous in that while the automobile is running, the antenna produces a whistling sound. Also, the antenna may be stolen. Still further, it is rather difficult to install the antenna, and handling the whip antenna is troublesome when a sub-antenna is added in a diversity receiving system.
In the case of a window glass plate antenna unit, the antenna element is installed inside the vehicle. Therefore, employment of the window glass plate antenna unit can substantially eliminate the above-described difficulties. However, the antenna has a rather poor directional pattern. In addition, the antenna is much lower in sensitivity than a whip antenna having a length of the order of 1/4 wavelength and which is installed outside the automobile.
The antenna unit installed on the rear window glass plate in which the antenna element also acts as the heating wire for the glass plate or the antenna unit in which the antenna element is close to the heating wire is disadvantageous in that it is liable to receive noise from electric power sources.
When an automobile receives an FM broadcast signal while travelling, it receives radio waves directly from the broadcasting antenna and radio waves reflected by the ground, buildings near the road, etc. Therefore, often multipath distortion is caused by the interference of these radio waves, as is well known in the art.
In order to eliminate the above-described difficulties, a signal receiving unit employing an antenna diversity system has been proposed and put to practical use. In such a signal receiving unit, a plurality of antennas are disposed at certain intervals with the directions of directivity being different from one another. In the signal receiving unit thus designed, the outputs of the plurality of antennas are monitored at all times and the antenna whose output is the highest is used, or when the output of the antenna presently in use decreases to a predetermined level, another antenna is used.
In the case of the above-described signal receiving unit of the antenna diversity system, at least two antennas must be provided. That is, in addition to the existing antenna, another antenna must be provided. In this case, it is desirable to use an antenna which is free from the above-described problems and is easy to install.
FIG. 19 shows a Smith chart on which a measured impedance characteristic of a whip antenna 90 cm in total length is drawn. The center of the chart (1.0) corresponds to the impedance (75 ohms) of the coaxial cable. The impedance of the antenna is a purely resistive 75 ohms substantially at the center frequency of the FM band.
FIG. 22 shows the directional pattern of a whip antenna when the latter receives vertically polarized waves, and FIG. 23 shows the directional pattern of the same antenna when the latter receives horizontally polarized waves.
FIGS. 10B, 11 and 12 show the impedance characteristics of the antenna measured by changing the length of the antenna element with the output part of the antenna secured to the upper left portion of the inner surface of the front window glass plate, as shown in FIG. 10A, and a copper foil 10 mm in width and 0.3 mm in thickness employed as the antenna element. As is apparent from these charts, the impedances are greatly shifted, and accordingly the antenna is low in sensitivity. In the case of FIG. 10B, the length of the antenna element was set to 1/4 wavelength (90.4 cm). In the cases of FIGS. 11 and 12, the antenna element lengths were 1/7 wavelength (51.6 cm) and 1/8 wavelength (45.2 cm), respectively.